Uptake and Release of Abscisic Acid by Runner Bean Root Tip Segments

Abstract

Uptake of abscisic acid by 5 mm long decapped root tips is a linear function of the external ABA concentration in the range of 2.9 × 10-8m to 10-4м and decreases dramatically with in­creasing pH. At pH 8.0 uptake rate is extremely low, even at high ABA concentrations. This indicated that nearly all of the ABA is taken up as the undissociated molecule ABAH. Uptake of ABA is influenced by agents modifying the pH gradients between the medium and the tissue such as salts of weak acids incubated at low external pH (inhibition of uptake and stimulation of ABA release by abolishing the pH gradients), protonophores such as CCCP (inhibition of uptake) and fusicoccin (stimulation of uptake by increasing the pH between medium and cytoplasm). It is concluded that ABA distributes between the compartments of the root cells according to the pH gradients with the undissociated molecule as the only penetrating species. Uptake and release occur without participation of a saturable component by diffusion. In contrast IAA permeates the plasmalemma as both IAAH and IAA-.